1. Field of the Invention
The present invention relates to a lead frame in which a metal layer such as a copper layer different in thickness is formed on both sides of an etching stop layer as an intermediate layer and leads are formed by the thinner metal layer of them, a method of manufacturing the lead frame, a semiconductor device using such a lead frame and a method of manufacturing the semiconductor device.
2. Description of the Related Art
There is technique for installing a semiconductor device using a lead frame provided with an etching stop layer in which a thick metal layer composed of copper for example to be an outside frame or an outside ring for example is formed on one side of the etching stop layer composed of aluminum for example and a thin metal layer composed of copper for example or patterned leads composed of the metal is/are formed on the other side, the metal layers are etched using the etching stop layer as an etching stopper to pattern the thick metal layer and the thin metal layer for forming leads and afterward, the etching stop layer is etched using the above both metal layers as a mask.
FIGS. 10A to 10E and FIGS. 11A to 11C show a method of manufacturing such a lead frame in the order of processes (A) to (H).
(A) Lead frame material 1 with three-layer structure shown in FIG. 10A is prepared. As for the lead frame 1, an aluminum film 3 approximately 3 .mu.m thick for example is formed on the surface of a substrate 2 approximately 150 .mu.m thick for example composed of copper or a copper alloy by vapor deposition or sputtering and further, an underplate layer 4 approximately 0.1 to 2 .mu.m thick for example composed of thin copper or nickel is formed. The lead frame material 1 is finally left as an outside ring.
(B) Next, a resist film is formed on the above underplate layer 4, the resist film is exposed and developed for patterning. The pattern of the resist film is the negative pattern of the pattern of leads to be formed. When the above patterning is finished, a film composed of copper or nickel and others is formed by plating using the resist film as a mask. A reference number 5 denotes a lead formed by plating and FIG. 10B shows a state after the resist film is removed after the lead 5 and others are formed.
(C) Next, as shown in FIG. 10C, a lead frame in which plural film circuits are integrated by selectively etching the lead frame material 1 from both sides so that the selective etching pierces a part of the lead frame material is formed. In the above etching, etchant such as ferric chloride is used.
(D) Next, as shown in FIG. 10D, an insulating layer (an insulating film) 6 is selectively formed on the surface on the lead formed side of the above lead frame material 1. Resin material provided with photosensitivity is used for the insulating layer 6 and a desired pattern is formed by applying, exposing and developing the insulating layer. A reference number 7 denotes an opening for exposing a part in which the ball electrode 8 of each lead 5 of the insulating layer 6 is to be formed and the insulating layer 6 is selectively etched so that the openings 7 are provided.
(E) Next, as shown in FIG. 10E, a solder ball 8 to be an external terminal is formed on the surface of the above lead 5 using the insulating layer 6 as a mask. The solder ball 8 is formed by nickel plate 80 to 110 .mu.m thick for example and solder or gold plate 0.1 to 5 .mu.m thick for example.
(F) Next, as shown in FIG. 11A, the inside from a part 9 to be an outside ring of the thick copper layer 2 located on the rear side of the lead frame material 1 is selectively etched from the rear side. In the above etching, etchant such as H.sub.2 SO.sub.4 and H.sub.2 O.sub.2 is used for example. The reason is that the above etchant erodes copper, however, it does not erode aluminum and an aluminum layer 3a can function as an etching stopper. In this process, the aluminum layer 3a is left except a part removed in the selective etching shown in FIG. 10C.
(G) Next, as shown in FIG. 11B, the underplate layer 4 under the leads 5 and the aluminum layer 3a as an etching stopper are etched using the above leads 5 and the left thick copper layer 2 as a mask. Hereby, each lead 5 is separated and is released from a state in which each lead is mutually electrically short-circuited.
(H) Next, if necessary, as shown in FIG. 11C, a bump 10 is formed at the end of each lead 5. Therefore, the bump may be formed on the side of the semiconductor device and no bump may be formed.
FIG. 12 is a sectional view showing a semiconductor device installed using the lead frame manufactured according to the method shown in FIGS. 10 and 11. A reference number 11 denotes a semiconductor device and each electrode of it is connected to the end of the above each lead 5 via the bump 10. A reference number 12 denotes sealing resin for sealing the semiconductor device 11, 13 denotes a thin dish-shaped heat spreader connected to each rear of the lead frame and the semiconductor device 11 and 14 denotes a conductive adhesive composed of silver paste for example for bonding the rear of the semiconductor device 11 to the heat spreader 13.
As for the above lead frame with multilayer structure in which a thick metal layer composed of copper and others is formed on one side of an etching stop layer composed of aluminum as an intermediate layer and a thin metal layer or leads composed of a thin metal layer is/are formed on the other side and a method of manufacturing it, various propositions are made by a company to which the applicants belong, however, there are the following problems in the technique.
First, to form the aluminum layer 3, vapor deposition or sputtering is required to be used, a high-priced device such as a vapor deposition device or a sputtering device is required to be used for vapor deposition or sputtering and there is a problem that such a high-priced device is the cause of the high cost. Second, bonding strength between a metal layer formed by vapor deposition or sputtering and a layer under the metal layer is weak independent of whether the metal layer is composed of aluminum or not and there is a problem that peeling between the aluminum layer and the copper film is readily caused in a process in which the lead frame is manufactured by the invasion of a chemical between the aluminum layer 3 and the copper film 2. The reason is that in case a metal layer is formed by vapor deposition or sputtering, bonding strength between layers depends upon van der Waals force and the force is remarkably weak, compared with adhesive strength in case a metal layer is formed by plating and others.
Particularly, a chemical readily invades between the copper film 2 of the outside ring 9 and the etching stop layer 3 composed of aluminum and peeling is readily caused. A problem that the outside dimension of a package is changed and resin for sealing, adhesion and others is cracked is caused by such delamination.